As shown in FIG. 1, a typical optic clamp uses a soft nylon tip set screw 10 to push the optic against two opposing lines of contact 20. The nylon tip set screw is prone to movement within its thread clearances, and the nylon tip is soft and deforms over time, both these factors affect the stability of the optical mount. The movement and deformation of the set screw results in movement of the optic, thus compromising the long term stability of the optical element.
The nylon tip set screw also has a low coefficient of friction, thus allowing unwanted movement of the optic under temperature cycling. Its low yield point allows the nylon tip to cold flow under forces normally used to retain the optic. These factors lead to the optic being held with a force that is insufficient for many optic holding applications.
Additionally, the nylon tip that contacts the edge of the optic is often of poor quality with the nylon tip contact surface varying from part to part. The nylon tip is difficult to control in terms of its mechanical tolerances after being press fit into the body of the set screw that supports it. The variable quality of the nylon tipped set screw adds unpredictability to the force holding the optic and unpredictability to the area of the contact between the nylon tip and the optic being secured. This variability makes it difficult to establish a pre-determined torque value that will achieve a repeatable optic surface flatness and holding force.
There are a number of issues that can be introduced when securing an optical element, for example the following three issues can arise; one the optical flatness can be compromised, two the susceptibility to vibrational forces can be made worse, and/or birefringence can be introduced into the optical element. And under extreme conditions the optical element can be destroyed. With this invention, the forces that hold the optical element can be precisely controlled, providing for a clear tradeoff between the holding force and the deformation of the optical element even over large temperature excursions.
Many of these problems were solved with the THORLABS POLARIS mirror mount model Polaris-K1, shown in FIG. 2. This mount utilizes a flat spring 30 that provides metal support between optic and the set screw. However, this design has some performance limitations.
First the flat spring was held in place with two setscrews and epoxy creating a stack up of components made from dissimilar materials that could cause some drift when exposed to extreme temperature variations.
Second the flat spring was prone to forming a dimple when excessive setscrew torque was applied to the locking setscrew; this dimple would create a single point of contact with the optic leading to high stress in the optic which can compromise its performance.
Even though the POLARIS-K1 design has improved security with which it holds the optic, this single point of contact creates a single point of high stress on the edge of the optic. This stress causes surface distortion of the face of the optic held within the POLARIS-K1. Even though the flat spring provided for a much higher optic push out force (the force required to forcible remove the optic when secured in place) because it did not yield like the nylon tipped set screws, it could only be lightly tightened in order to prevent excessive optical distortion as can be seen from the graph of optical distortion in FIG. 3. This figure shows the variation in optical distortion or flatness of a 6 mm thick mirror mounted within the POLARIS-K1 Mirror Mount offered by Thorlabs Inc. The recommended torque is 6-10 inch-ounces for a 6 mm Thick UVFS Laser Quality Mirror as this keep the optical surface distortion below 0.1 waves.
The 10 inch ounces torque on the first generation POLARIS product provides for about 1 lbf of optic axial push out force before the optic begins to move out of its bore. While this in an improvement over the classic nylon tipped set screw design that begins to yield at a much lower push out force, the new monolithic flexure design can increase the push out force to well over 10 lbf while still keeping the surface flatness of the optic at or below 0.1 waves of flatness. This is because with the nylon tipped set screw design the stresses of the nylon material cause the material to cold flow and yield, allowing the optic to move in its bore. The only way to increase the holding force with the nylon tipped set screw design is to increase the clamping force, but increasing the clamping force over stresses the optic causing surface distortion and birefringence.
With the first generation POLARIS flexure spring design the thin flat spring yields and dimples creating a small contact point. This small surface contact creates high stresses when exposed to an axial load and the material exceeds its yield point, allowing the optic to move. With the monolithic flexure design the optic contact reign is thick enough not to yield under preload allowing for a ridged line of contact as opposed to a point of contact. This contact line distributes the stresses over a greater area allowing for a much higher axial load before the material begins to yield. Although the typical nylon tipped set screw optic retention may be sufficient for many controlled laboratory applications it is not considered sufficient for many industrial applications outside a well-controlled lab environment.
It is thus desirable to have a solution that would eliminate the stack up of component joints, the stack up of dissimilar materials, the single point high stress optic contact and also provide a stronger more ridged structure to better hold the optic in place within an optical mount.
It is also desirable to be able to isolate the high stresses of the set screw, moving them away from the optic contact line, preserving the precision machined surface of the contact line and also providing a degree of isolation of the forces created by thermal expansion and contraction of the optic mount.
Therefore there is a need for a new optical mount design that would address all these concerns.